I never read all the way through a Kindle book, but my wife just did - and told me that at the end of the book you can give it a star rating. It says that the rating will go under her Amazon screen name. She added that there wasn't a written review option, only a star rating.
It's data to help Amazon refine their marketing to that customer. The rating might appear under the customer's username on Amazon (for instance, if a text review is left on site and another customer clicks that username to see what else has been reviewed), but it's primarily to benefit the recommendation machine.